A. Field of the Invention
The invention relates to medical diagnostics and, more particularly, to assessment of the venous circulation of human limbs, particularly the legs.
B. Prior Art
The human circulatory system is susceptible to numerous diseases and disorders. Frequently, these disorders manifest a defect in the circulatory system itself, while in other cases the disorder reflects a defect of primary origin elsewhere. In either case, it is desirable to be able to rapidly and accurately assess the functioning of the circulatory system of the patient.
The venous system of humans is particularly susceptible to seriously debilitating disorders arising from the gradual build-up of blood clots within the veins. In addition to obstructing the venous return to the heart, these clots sometimes reach a substantial size, break off, and then travel to the heart and lungs where they block circulation and frequently cause death. This complication can be prevented if clots are detected and appropriate treatment is scheduled. Thus, it is highly desirable to be able to determine whether one or more of these clots are present in the veins.
One technique I have heretofore utilized to assess venous patency is to measure the increase in venous volume accompanying a forced blockage of the venous return to the heart. This blockage may be applied by causing the patient to inhale deeply, thereby increasing intraabdominal pressure which in turn restricts venous return, or by applying a force to the venous system by means of devices such as a pressure cuff which collapses the veins and thereby blocks off the venous return. When the venous return is blocked in this manner, the veins accumulate the blood pumped into them from the arteries and store the excess blood by expanding their cross section to thereby increase the stored volume. The increase in blood volume (hereinafter termed "venous capacitance") is a measure of the distensibility of the veins and thus of the condition of the venous system.
Heretofore I have found that patients having a venous blockage such as is occasioned by a blood clot exhibit a noticably lower volume increase accompanying a forced blockage of the venous return than patients whose veins are normal. In particular, I measured the change in impedance through a portion of the limb being tested in response to a forced blockage of the venous return to the heart from that limb during deep breathing and compared it to the impedance measured when the patient was resting and absent any forced blockage, that is, the "resting baseline impedance." I found that patients exhibiting an impedance change of less than 0.2% of the resting baseline impedance in response to a forced blockage generally had one or more blood clots in their veins, while patients exhibiting an impedance change of greater than this amount were generally free of clots. This test has been extremely useful and beneficial; however, in certain cases it produces incorrect results. For example, a patient who is hypovolemic (that is, having a low blood volume) exhibits a low volume change on blockage of the venous return, thereby indicating a blood clot in the veins when actually none is present. Additionally, the measurements obtained frequently varied somewhat from observer to observer depending on judgemental factors of the observer in administering the test. Many sick or debilitated patients are unable to cooperate with the breathing maneuvers required. Thus, a test of greater accuracy and applicability is needed.